The detection of the level of glucose or other analytes, such as lactate, oxygen or the like, in certain individuals is vitally important to their health. For example, the monitoring of glucose is particularly important to individuals with diabetes. Diabetics may need to monitor glucose levels to determine when insulin is needed to reduce glucose levels in their bodies or when additional glucose is needed to raise the level of glucose in their bodies.
Devices have been developed for continuous or automatic monitoring of analytes, such as glucose, in bodily fluid such as in the blood stream or in interstitial fluid. Some of these analyte measuring devices are configured so that at least a portion of the devices are positioned below a skin surface of a user, e.g., in a blood vessel or in the subcutaneous tissue of a user.
Following the sensor insertion, the resulting potential trauma to the skin and/or underlying tissue, for example, by the sensor introducer and/or the sensor itself, may, at times, result in instability of signals monitored by the sensor. This may occur in a number of analyte sensors, but not in all cases. This instability is characterized by a decrease in the sensor signal, and when this occurs, generally, the analyte levels monitored may not be reported, recorded or output to the user.
Proper calibration of an analyte sensor with a reference glucose measurement or reading is important for accurate sensor performance. Calibration is a process by which a conversion factor (or sensitivity) is determined and represented, in its simplest form, as a ratio of the electrical current generated by the analyte sensor to the reference blood glucose value (for example, from an in vitro blood glucose meter) associated in time (for example, relatively time corresponding) with the current signal from the analyte sensor. Ideally, the sensitivity is constant throughout the life of the analyte sensor when positioned in fluid contact with an analyte of a user (such as interstitial fluid). In practice, however, the sensitivity may vary over time. It has been observed that a depression or attenuation in the sensitivity, usually following a predetermined time period measured from the insertion or positioning of the analyte sensor, occurs sometimes for up to 24 hours for some analyte sensors. This signal characteristic is referred to as Early Sensitivity Attenuation (ESA) or referred to as ESA condition. The ESA condition may be a result of a physiological response to the introduction of the analyte sensor to the subcutaneous tissue, and may be present for any subcutaneously inserted analyte sensor.
Generally, the use of a standard calibration sensitivity calculation does not address the signal attenuation. A typical standard calibration does not detect or manage the attenuated signal characteristics, and also may potentially update or modify the calibration sensitivity using the erroneous and attenuated sensor signal. When sensor calibration is performed while the sensor is undergoing a signal attenuation event, the reported or resulting sensor data may be erroneously high when the sensor sensitivity has recovered after the termination of the signal attenuation event. Such high biased results may be clinically unsafe, as they may lead to missed hypoglycaemic events, or overdoses of medication such as insulin. On the other hand, when sensor calibration is performed prior to an early signal attenuation event, erroneously low biased sensor data will likely result during the period of the sensor sensitivity depression. Such low glucose results may, depending on the magnitude of the early signal attenuation event, result in false hypoglycaemia alarms or missed hyperglycaemic events.
Another approach has been to delay the sensor calibration until after the early signal attenuation period measured, for example, from the initial sensor insertion in the patient. However, this approach prevents the reporting of the potentially erroneous analyte level monitored from the sensor during this period, but results in low data yield due to the undesirable delay for the display or reporting of the monitored analyte levels from the sensors regardless of whether or not early signal attenuation is present.